General vehicles and mobile robots are divided into the independent two driving wheel mechanism and the steering and driving mechanism according to the driving mechanism. The present driving mechanisms mentioned above have the limited movement, that is, they can not instantaneously move in the side direction without rotating directly the body of vehicle in a state of forward movement. Because of this limitation of movement, it is difficult to move in a narrow space and it is necessary to calculate the complex path in order to reach a destination.
The omni-directional driving mechanism was devised to overcome the aforementioned defects. The omni-directional driving mechanism can instantaneously move in any direction in any state of motion by making it possible to have 3 degree of freedom motion (longitudinal, lateral and rotational).
For the omni-directional driving, several types of omni-directional mechanism were proposed and, in general, they are classified into two methods. The one is the adoption of conventional wheels. The other is the adoption of specially designed wheels for the omni-directional driving. Off-centered wheel mechanism, which is a typical mechanism of conventional wheels, has steering axles installed off-centered from each wheel center at each wheel so that it can be omni-directionally driven by steering and driving each wheel. There are Universal wheel, Mechanum wheel, Double wheel, Alternate wheel, Half wheel, Orthogonal wheel and Ball wheel among the specially designed wheels for omni-directional driving. Their shapes are different each other but they commonly have the active mode of transferring the driving force through the rotation of wheels and the passive mode of free rotation without transferring the driving force. Wheels which are applied to the omni-directional driving with the aforementioned active and passive mode are referred to as the omni-directional wheels.
FIG. 1 shows a schematic configuration of omni-directional wheels. Universal wheels shown at (a) of FIG. 1 is the most traditional among omni-directional wheels and comprises several passive rollers which rotate freely at each axis. Here, the rotation axis of each passive roller is tangent to the wheel circumference respectively. But the discontinuous contact with the ground resulted from the gap between the passive rollers makes a Universal wheel applied vehicle be vertically vibrated.
Mechanum wheel and Double wheel shown at (b) and (c) of FIG. 1 respectively were proposed to remove the vertical vibration. Each roller of the Mechanum wheel is arranged at the angle of 45 degrees with the wheel axis and the Double wheel is overlapped with two wheels. But, their translational operation results in the horizontal vibration because they are also in discontinuous contact with the ground. And, even though the wheels rotate with a constant angular velocity, the turning speed of the Mechanum wheel or the Ball wheel applied vehicle is not constant during its turning operation because the distance between the wheel and the center of vehicle changes at the existence of discontinuous contact. Furthermore, the Mechanum wheel functions as a decelerator so that it makes an effect of the increase of inertia, which means that the operation efficiency of the Mechanum wheel applied vehicle is decreased.
The Alternate wheel and Half wheel shown at (d) and (e) of FIG. 1 respectively were proposed to minimize the horizontal and vertical vibration. The Alternate wheel has the shape that small and large rollers are arranged by turns and the half wheel has the shape that half cut rollers are serially overlapped.
The Orthogonal wheel shown at (f) of FIG. 1 has two roller of which axes of rotation are perpendicular each other and the Ball wheel shown at (g) of FIG. 1 embodies the active and passive mode with the sphere type wheel.
The aforementioned omni-directional wheels have shortcomings respectively but, contrary to conventional wheels, they have the driving torque transferring direction by the active mode and the external force driven free rotation direction by the passive mode so that they have the advantage of 3 degree of freedom motion (longitudinal, lateral and rotational) at a two dimensional space.
In regard to this, Roger F. Hiscock proposed an omni-directional wheel applied toy vehicle on American U.S. Pat. No. 4,335,899.
FIG. 2 shows a schematic configuration of the toy vehicle of the aforementioned patent.
Referring to FIG. 2, the toy vehicle 10 comprises a frame 15 and a seat 20 located at the rear upper part of the frame 15. Rear wheels 25 installed at the rear part of the frame 15 are rotatable by the driving axle 30 and the housing 35 covers the supporting axle 45 in front of the frame 15. Multiple rollers 70 which are rotatable in the direction perpendicular to the direction of rotation of the rear wheels 25 are arranged at the circumference of the rear wheels 25. Also, the front wheels 40 installed in front of the frame 15 are rotatable by the supporting axle 45 and connected to the housing 35 via the pin 50 so that the toy vehicle 10 can be steerable. That is, the front wheels 40 are steered by the handle 55 and, in detail, by the steering axle 60 connected to the handle 55 in connection with the steering link engaged with the steering axle 60. The pedals 65 are located near the housing 35 and connected to the driving axle 30 via a chain or a sprocket so that their operation can drive the rear wheels 25.
But, even though the toy vehicle 10 adopts the omni-directional wheel of universal wheel type for the rear wheels, it can not perfectly move in all directions because two rear wheels 25 are not operated independently as well as the toy vehicle is steered by steering the front wheels 40 connected to the handle 55.